1. Field of the Invention
The present invention relates to an improved high resolution system for electrically sensing the spatial coordinates of a electronic point specifying device such as a stylus ("stylus" is used herein to describe the hand-held probe or other point specifying device) with respect to a conductive two-dimensional coordinate system independent of variations of the stylus in the third orthogonal dimension. Using a high precision input signal, precision signal processing, and by removing stochastic and deterministic noise, the preset invention improves point sensing precision and accuracy and hence the precision and accuracy of the spatial coordinates calculation.
2. Description of Prior Art
The present invention is an improvement over the prior art as disclosed in U.S. Pat. No. 4,603,231 (the "'231 Patent"), issued to L. Reiffel, et al., for "System for Sensing Spatial Coordinates", which is hereby incorporated by reference. The present invention will be described by way of reference to its distinguishments from the '231 Patent.
The present invention provides significant improvements in precision over the prior art as described in the '231 Patent. Substantial improvements in the precision of the sensed coordinates is achieved by substantial removal of stochastic noise signals from the information signal. By removing noise from the information signal, the precision of the present invention is improved over the prior art.
In sensing the position of a coordinate, the prior art system collected information by processing the output of a full-wave rectifier with a low-pass "time-averaging" filter. The low-pass filter weighted the input signal with respect to time such that the most recent signal input had greater weight than the previous signal. The weight of the signal at each instant in time during the sampling period has unacceptably large variance. Although this system was highly precise with respect to its constituent components and the display devices available at the time of its conception, the prior art system cannot provide the precision available with current components and display devices such as high resolution monitors.
The present invention implements a precision integrating system that integrates equally weighted periods of time and averages these periods of time such that all input information has equal weight. The final value of the integration is not dependant on the sequence of input events, only the magnitude and quantity of the events. In contrast with the time averaging or low-pass filtering system of the prior art, in which later events weigh more heavily than earlier events, the integrator of the present invention provides improved performance over the low-pass "time-averaging" filter method since the weight of each sample is proportionate to the sample interval. The present invention's signal processing method maintains the integrity of the input signal and reduces noise, thereby improving the precision of the coordinate sensing.
Additionally, the hand held stylus used in prior art systems as an input device was inadequate for the high resolution capabilities of the present invention. Therefore, the stylus, a functionally dependent component of the present invention, provides precision sensing of coordinate location equal to the overall electronic performance of the present invention.